1. Field of the Invention
The present invention relates to a shadow mask support frame for a color cathode ray tube (CRT), in which a shadow mask is supported with tension.
2. Background of the Related Art
Generally, a shadow mask support frame for a color CRT supports a shadow mask that acts to selectively pass through an electron beam, so as to prevent the shadow mask from being oscillated due to external shock or sound wave. The shadow mask support frame also supports the shadow mask to maintain a predetermined distance from a fluorescent screen to land the electron beam passed through an electron beam passing hole of the shadow mask on a predetermined phosphor.
FIG. 1 is a partially exploded side view of a related art color CRT incorporating a shadow mask support frame. Referring to FIG. 1, the related art color CRT includes a flat panel 1 on which a dot type fluorescent screen 2 having R, C, B colors is coated, a shadow mask 4 having an electron beam passing hole 4 a that acts to selectively pass through an electron beam 3 incident upon the fluorescent screen 2, a funnel 5 coupled to the rear of the panel 1, a neck portion 5a formed at the rear of the funnel 5, an electron gun (not shown) mounted into the neck portion 5a, for emitting the electron beam, a deflection yoke 6 for deflecting the electron beam surrounding an outside of the funnel 5, a shadow mask support frame 7 for supporting the shadow mask 4, a spring 8 mounted in the shadow mask support frame 7, a stud pin 9 fixed into the panel 1 to be coupled to the spring 8, for supporting the shadow mask support frame 7, and an inner shield 10 formed at the rear of the shadow mask support frame 7, for shielding earth-magnetic field so as not to allow the electron beam 3 to be susceptible to external earth-magnetic field when the CRT is operating.
The operation of the aforementioned color CRT will be described.
The electron beam emitted from the electron gun is horizontally/vertically deflected by magnetic field of the deflection yoke 6 and scanned on the shadow mask 4. Then, the electron beam selectively passes through the electron beam passing hole 4a of the shadow mask 4. At this time, the electron beam passing hole 4a of the shadow mask 4 and the fluorescent screen 2 coated on the flat panel 1 are set to geometrically match with each other. Accordingly, when the electron beam 3 passes through the electron beam passing hole 4a, the electron beam 3 is landed on the fluorescent screen 2 to emit light on the fluorescent screen 2. A picture image is displayed by emitting light on the fluorescent screen 2. The picture image can be displayed with a predetermined color purity only when the electron beam 3 is accurately landed on the fluorescent screen 2.
However, when the electron beam 3 passes through the electron beam passing hole 4a of the shadow mask 4, the shadow mask 4 emits heat so that the shadow mask 4 is thermally expanded. This is called a doming phenomenon.
Afterwards, heat of the shadow mask 4 is transferred to the as shadow mask support frame 7 so that the shadow mask support frame 7 is thermally expanded. This results in that the electron beam passing hole 4a of the shadow mask 4 is deviated from a preset position. Therefore, even if the electron beam 3 passes through the electron beam passing hole 4a, the electron beam 3 fails to be landed on the predetermined fluorescent screen 2, thereby causing color purity on a screen any adverse effect.
Meanwhile, if outwardly provided oscillation or impact is applied to the shadow mask support frame 7 and the shadow mask 4 through the panel 1 and the stud pin 9, a howling phenomenon of the shadow mask 4 occurs. The howling phenomenon moves the position of the electron beam passing hole 4a of the shadow mask 4, thereby causing color purity any adverse effect in the same manner as the doming phenomenon.
Moreover, as a curvature radius of the shadow mask becomes flat recently, the shadow mask has a poorer rigidity than a spheric shadow mask. This seriously causes the doming phenomenon and the howling phenomenon.
To solve such a problem, as shown in FIGS. 2 to 5, there are provided shadow mask support frames according to first to third embodiments of the related art, in which tension is applied to the shadow mask 4 to absorb thermal expansion of the shadow mask due to the doming phenomenon and reduce oscillation due to the howling phenomenon.
Referring to FIGS. 4 to 6, the related art shadow mask support frame 7 includes two main frames 71 arranged in a row, and two sub frames 72 arranged in a row to be orthogonal to the main frames 71, having both end portions fixed to respective end portions of the main frames 71. Thus, the shadow mask support frame 7 has a square frame shape.
The aforementioned shadow mask support frame 7 is compressed in a length direction (Y direction) of the sub frame 72 by a separate compression equipment. In this state, both sides of the shadow mask 4 are fixed on the main frame 71, and then compression load is eliminated. Thus, the main frame 71 and the sub frame 72 are restored to their original positions by elastic force. As a result, the shadow mask 4 is subject to tension in Y direction.
Since the tension applied to the shadow mask absorbs thermal expansion of the shadow mask 4, the doming phenomenon can be avoided to some extent. However, the tension applied to the shadow mask could not completely remove the howling phenomenon due to structural characteristic of the shadow mask support frame 7 as described later.
The shadow mask support frames 7 according to the first to third embodiments of the related art have the frequency of primary unique oscillation, inertia moment values, compression load values, and weight values of the shadow mask support frame 7, respectively, as shown in Table 1 below.
The frequency X of unique oscillation of the shadow mask support frame 7 is expressed by the following equation.   ω  =                    K        ρ              ∝                            E          ⁢                      xe2x80x83                    ⁢          I                          ρ          ⁢                      xe2x80x83                    ⁢                      l            4                              
Where, xcfx89 is the frequency of unique oscillation of the support frame, K is frame rigidity, xcfx81 is a density (frame mass), E is elastic coefficient, I is inertia moment, and L is a frame length.
In the above equation, the following relationship is obtained.
xcfx89xe2x88x9dIxe2x88x9dK
In the support frame structure having the same material, the same length, and the same elastic coefficient, if the inertia moment increases, rigidity increases with the frequency of unique frame oscillation. If the frequency of unique frame oscillation increases, the resonation range to external oscillation having a certain area or impact is reduced, thereby improving howling characteristic.
However, in the first embodiment of the related art as shown in FIGS. 2A to 2C, supposing that the shadow mask support frame 7 has a characteristic value of 100%, there is a problem that compression load and weight required for tension of the shadow mask 4 increase.
Furthermore, in the second embodiment of the related art as shown in FIGS. 3A to 3C, the shadow mask support frame 5 has a high inertia moment value for the sub frame 72 but a low inertia moment value for the main frame 71. This causes twist moment to be weakened, thereby causing poor howling characteristic.
In the third embodiment of the related art disclosed in the Japanese Patent Publication No. 5-258677, as shown in FIG. 4, the main frame 71 has a triangle shaped hollow section and the sub frame 72 has a solid section. Since the sub frame 72 has end portions bent orthogonally, the frequency of primary unique oscillation and the inertia moment of the main frame 71 can be improved. However, the weight of the shadow mask support frame 5 increases, thereby reducing unique oscillation characteristic.
Accordingly, the present invention is directed to a shadow mask support frame for a color CRT that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a shadow mask support frame for a color CRT in which a coupling portion between a main frame and a sub frame, and a shape of the sub frame are improved to increase the frequency of unique oscillation, thereby reducing a howling phenomenon occurring in a shadow mask.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a shadow mask support frame for a color CRT according to the present invention includes: two main frames having hollow sections; two sub frames having solid sections, end portions respectively fixed into respective end portions of the main frames, a support portion inwardly extended with a predetermined length at an obtuse angle to the end portions, and a central portion inwardly extended with a predetermined length at an obtuse angle to the support portion; and a bracket mounted between respective end portions of the main frames and the sub frames, for reinforcing fixing portions of the main frames and the sub frames.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.